When two materials slide against each other, it is often desirable to have a smooth contact between the surfaces of the materials so to reduce friction at the contact area. For a given pair of sliding surfaces, the magnitude of friction is generally given by the coefficient of friction. A lowering of the coefficient of friction generally leads to improved wear and reduced squeaking noises.
An important area of application where smoothly sliding materials are needed is in drug injection systems, which are commonly used to deliver therapeutic agents. Pen injectors constitute one class of drug injection systems. Pen injection components are commonly made of plastic materials, such as thermoplastics. The components are generally produced by injection moulding techniques. Pen injectors are widely used for delivering or injecting therapeutic agents into the body, example, a human body. Surfaces of a pen injection system that are in contact with each other slide during injection of the therapeutic agent into the human body. This leads to friction between the surfaces of the components that are in contact during the sliding motion. For example, the outer surface of a piston experiences sliding friction when it slides against a cylindrical tube of a pen injection system. The force with which a therapeutic agent is injected into a human body through a pen injection system is termed as dose force. One of the factors that determines the dose force in the pen injector is the friction between components of the pen injector that slide against each other. Therefore, if friction can be reduced, the dose force can also be reduced advantageously, thereby leading to a smoother operation and longer use of the injection system.
The known pen injector components exhibit a relatively high coefficient of friction when they slide against each other. Further, the components manufactured by commonly known techniques, such as injection moulding, require extensive running in, external lubrication, or both to obtain an instantaneous low coefficient of friction between them.
Therefore, there is a need for a system having components having a low coefficient of friction, that is, a coefficient of friction of less than 0.06 when measured using a contact pressure of 3.0 MPa (megapascals) and a sliding speed of 0.02 meters per second. Further, there is a need for methods for producing a low friction system that exhibits a low coefficient of friction of less than or equal to 0.06, without subjecting the components to the extra steps of exhaustive running-in, external lubrication, or both. Furthermore, there is a need for a system wherein after additional external lubrication of one or both components' surface(s), the frictional force is not further reduced.